fix(desktop): retain backend worktree project ownership

apps/desktop/src/app/chat/sidebar/index.tsx

@@ -1045,8 +1045,15 @@ export function ChatSidebar({
   // overlay always has a lane to place a missing in-project session into.
   const enteredProjectContent = useMemo(
     () =>
-      enteredProject ? overlayLiveLanes(enteredProject, enteredProjectOverlaySessions, removedSessionIds) : undefined,
-    [enteredProject, enteredProjectOverlaySessions, removedSessionIds]
+      enteredProject
+        ? overlayLiveLanes(
+            enteredProject,
+            enteredProjectOverlaySessions,
+            removedSessionIds,
+            projectOwnerBySessionId(agentProjectTree)
+          )
+        : undefined,
+    [enteredProject, enteredProjectOverlaySessions, removedSessionIds, agentProjectTree]
   )
 
   const scopedRepoPaths = useMemo(

apps/desktop/src/app/chat/sidebar/projects/index.ts

@@ -15,6 +15,7 @@ export {
   liveSessionProjectId,
   overlayLiveLanes,
   overlayLivePreviews,
+  projectOwnerBySessionId,
   reconcileEnteredProjectSessions,
   sessionBucketId,
   sessionMatchesProjectFilter,

apps/desktop/src/app/chat/sidebar/projects/workspace-groups.test.ts

@@ -13,6 +13,7 @@ import {
   NO_PROJECT_ID,
   overlayLiveLanes,
   overlayLivePreviews,
+  projectOwnerBySessionId,
   reconcileEnteredProjectSessions,
   sessionMatchesProjectFilter,
   sessionProjectColor,
@@ -615,6 +616,36 @@ describe('sessionProjectColor', () => {
 })
 
 describe('overlayLiveLanes', () => {
+  it('does not inject a backend-owned sibling worktree session into an ancestor project', () => {
+    const sibling = makeCwdSession('/work/repos/app-2/src', { id: 'sibling', git_repo_root: null })
+
+    const ancestor = projectNode({
+      id: 'p_work',
+      path: '/work',
+      repos: [{ id: '/work', label: 'work', path: '/work', groups: [], sessionCount: 0 }]
+    })
+
+    const repo = projectNode({
+      id: 'p_app',
+      path: '/work/repos/app',
+      repos: [
+        {
+          id: '/work/repos/app',
+          label: 'app',
+          path: '/work/repos/app',
+          groups: [lane({ id: '/work/repos/app-2', label: 'app-2', path: '/work/repos/app-2', sessions: [sibling] })],
+          sessionCount: 1
+        }
+      ],
+      previewSessions: [sibling],
+      sessionCount: 1
+    })
+
+    const overlaid = overlayLiveLanes(ancestor, [sibling], new Set(), projectOwnerBySessionId([ancestor, repo]))
+
+    expect(overlaid.sessionCount).toBe(0)
+  })
+
   it('keeps an overview preview visible when the hydrated drill-in is stale', () => {
     const staleHistory = makeCwdSession('/www/app', {
       id: 'stale-history',
@@ -1085,6 +1116,19 @@ describe('overlayLiveLanes', () => {
 })
 
 describe('overlayLivePreviews', () => {
+  it('keeps a backend-owned sibling worktree session out of an ancestor preview when git_repo_root is null', () => {
+    const sibling = makeCwdSession('/work/repos/app-2/src', { id: 'sibling', git_repo_root: null })
+    const ancestor = projectNode({ id: 'p_work', path: '/work', previewSessions: [], sessionCount: 0 })
+    const repo = projectNode({ id: 'p_app', path: '/work/repos/app', previewSessions: [sibling], sessionCount: 1 })
+
+    const previews = overlayLivePreviews([ancestor, repo], [sibling], [
+      makeProject('p_work', ['/work']),
+      makeProject('p_app', ['/work/repos/app'])
+    ], 3)
+
+    expect(previews.p_work).toBeUndefined()
+    expect(previews.p_app?.map(session => session.id)).toEqual(['sibling'])
+  })
   it('merges live sessions into a project preview, live first, capped to the limit', () => {
     const project = projectNode({
       id: '/www/app',

apps/desktop/src/app/chat/sidebar/projects/workspace-groups.ts

@@ -476,6 +476,30 @@ export function sessionProjectColor(session: SessionInfo, projects: ProjectInfo[
   return projects.find(project => project.id === projectId)?.color ?? null
 }
 
+/**
+ * Membership in a project-tree snapshot is backend-resolved: in particular,
+ * the git probe can identify a sibling worktree even while its persisted row
+ * has not yet been backfilled with `git_repo_root`. Keep that answer when the
+ * live cache refreshes the same row instead of re-inferring ownership from its
+ * cwd (which can make an umbrella project claim it as well).
+ */
+export function projectOwnerBySessionId(projects: SidebarProjectTree[]): ReadonlyMap<string, string> {
+  const owners = new Map<string, string>()
+
+  for (const project of projects) {
+    const sessions = [
+      ...(project.previewSessions ?? []),
+      ...project.repos.flatMap(repo => repo.groups.flatMap(group => group.sessions))
+    ]
+
+    for (const session of sessions) {
+      owners.set(session.id, project.id)
+    }
+  }
+
+  return owners
+}
+
 const upsertSession = (rows: SessionInfo[], session: SessionInfo): SessionInfo[] =>
   [session, ...rows.filter(row => row.id !== session.id)].sort((a, b) => sessionRecency(b) - sessionRecency(a))
 
@@ -746,7 +770,8 @@ export function excludeProjectSessions(
 export function overlayLiveLanes(
   project: SidebarProjectTree,
   live: SessionInfo[],
-  removed: ReadonlySet<string> = NO_REMOVED
+  removed: ReadonlySet<string> = NO_REMOVED,
+  authoritativeOwners: ReadonlyMap<string, string> = new Map()
 ): SidebarProjectTree {
   if (project.isNoProject) {
     return overlayHomeLane(project, live, removed)
@@ -754,8 +779,14 @@ export function overlayLiveLanes(
 
   let changed = false
 
+  const projectLive = live.filter(session => {
+    const owner = authoritativeOwners.get(session.id)
+
+    return !owner || owner === project.id
+  })
+
   const repos = project.repos.map(repo => {
-    const next = overlayRepoLanes(repo, live, removed)
+    const next = overlayRepoLanes(repo, projectLive, removed)
 
     changed ||= next !== repo
 
@@ -804,13 +835,14 @@ export function overlayLivePreviews(
   { removed = NO_REMOVED, rankIds }: PreviewOverlayOptions = {}
 ): Record<string, SessionInfo[]> {
   const byProject = new Map<string, SessionInfo[]>()
+  const authoritativeOwners = projectOwnerBySessionId(projects)
 
   for (const session of live) {
     if (removed.has(session.id)) {
       continue
     }
 
-    const projectId = sessionBucketId(session, explicitProjects)
+    const projectId = authoritativeOwners.get(session.id) ?? sessionBucketId(session, explicitProjects)
 
     if (!projectId) {
       continue